The Candida phototoxicity test. The sensitivity of different strains and species of Candida, standardization attempts and analysis of the dose-response curves for 5- and 8-methoxypsoralen.

The purpose of this study was to answer the question as to whether PUVA-sensitivity of different Candida strains and species varies, to investigate the possibilities for standardization of the test and to produce calibration curves suitable for an approximate quantification of psoralens in plants. In the test, paper discs with different concentrations of 5- or 8-MOP in ethanol 60% were placed on yeast-seeded Sabouraud-agar dishes that were irradiated with UVA. The results were based on the diameter of growth inhibition zones. Small but significant differences in sensitivity were found between the 12 Candida strains tested, somewhat greater between the 15 species. Standardization of the method implies the use of the same test organism, depth of agar, time of pre-diffusion, medium, solvent and method for application of psoralen-extract and UVA dose. Dose-response curves for 5-MOP and 8-MOP obtained under standardized conditions demonstrated a linear relationship between the diameter of growth inhibition zones and the logarithm of the psoralen concentration in the range from 1 to 400 micrograms/ml. They were suitable for estimating the content of ethanolsoluble phototoxic substances in terms of 5- or 8-MOP equivalences.